Recently, the amount used for synthetic fiber paper obtained by a web forming method using polyethylene terephthalate fibers for a part or the whole of a material of paper has been increasing owing to its excellent physical characteristics such as mechanical characteristics, electric characteristics, heat resistance, dimensional stability, hydrophobic nature and the like and cost advantages. As a binder fiber used for the synthetic fiber paper, polyethylene fibers and polyvinyl alcohol fibers were used in the past but polyethylene terephthalate fibers are mainly used at present. For the synthetic fiber paper mainly using polyethylene terephthalate fibers, the same kind of polyethylene terephthalate fibers are mainly used as an optimal binder. Moreover, in recent years, in the fields of heat-retaining materials, electrical insulating materials, filters, medical materials, construction materials and the like, a demand for development of wet-laid nonwoven fabrics having heat resistance has become high. Thus, a wet-laid nonwoven fabric formed of fibers using polyethylene naphthalate which is one of polyesters having higher heat resistance as a material has been developed (See Patent Literature 1, for example).
However, depletion of petroleum and wood has become a serious social problem in recent years, and sustainable development is given importance. Thus, a wet-laid nonwoven fabric using a polylactic acid fiber which is a biomass-derived component is proposed (See Patent Literature 2, for example). However, with such a wet-laid nonwoven fabric, the melting point of polylactic acid which is a polymer is as low as in the vicinity of 170° C., hydrolyzability is low, and fully satisfactory values of adhesive strength and heat resistance of the wet-laid nonwoven fabric have not been obtained.
Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2009-221611
Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2010-180492